1.Field of the Invention
The present invention relates to a device for manufacturing a preform for optical fibres through chemical deposition on a substrate for deposition arranged vertically. More specifically, the invention relates to a device for manufacturing one or more preforms for optical fibres through a chemical deposition process on one or more substrates for deposition arranged vertically.
2.Description of the Related Art
As known, the methods for manufacturing optical fibre basically comprise a first process of manufacturing a preform from glass and a successive process of drawing the optical fibre from the preform.
The most common processes of manufacturing preforms comprise one or more chemical deposition steps, through one or more burners, of suitable chemical substances on a cylindrical support; the chemical deposition substances typically comprise silicon and germanium, which are deposited in the form of oxides (SiO2 e GeO2).
The processes of manufacturing preforms through chemical deposition known in the art comprise processes of the VAD (Vapor Axial Deposition) type and processes of the OVD (Outside Vapor Deposition) type.
Typically, in VAD type processes the cylindrical support is held in a vertical position by a gripping member which operates on an upper end of the cylindrical support; the cylindrical support is made to turn upon itself so as to expose its entire surface to one or more burners which are housed near to the lower end of the support and in such a position as to emit a flow of reactants along a direction which is inclined at a predetermined angle, typically lying between 30° and 50°, with respect to the longitudinal axis of the support. The support is then moved upwards so as to allow substantially axial growth of the preform.
In processes of the OVD type, on the other hand, the cylindrical support is held in a horizontal or vertical position by a pair of gripping members which operate on the opposite ends of the support; the support is made to turn upon itself so as to expose its entire surface to one or more burners mounted on a side of the support and in such a position as to emit the flow of reactants along a direction which is substantially perpendicular to the longitudinal axis of the support. The burner, in particular, is mounted on a support structure equipped with a motorised driving member which allows the repeated movement of the burner parallel to the cylindrical support, so as to allow a substantially radial growth of the preform along all the sections of the support.
A typical process of the OVD type comprises the following steps. In a first step a substantially cylindrical glass preform, called “core preform”, is manufactured through deposition of the chemical substances on the cylindrical support: such a preform is named in such a way since it will create the core and a more internal portion of the optical fibre's cladding.
In a second step, the cylindrical support is taken out of the core preform, freeing up a central hole in the preform.
In a third step, the core preform undergoes a process of desiccation and compacting in a furnace, during which suitable gases (comprising, for example, Cl2) are made to flow inside the central hole in order to eliminate the hydroxide ions (—OH) and the atoms of water present in the preform, thus obtaining a vitrified core preform which exhibits a central hole having a smaller diameter than that of the initial preform.
In a fourth step, after having created the vacuum inside the hole, the vitrified core preform is placed in a vertical furnace in which the melting of a lower end of the preform itself is carried out. Such a melting causes the walls of the hole to collapse due to the vacuum created inside of it; the glass material cools down to form an elongated cylindrical element of a predetermined diameter, which is pulled downwards by a suitable traction device. Such an elongated cylindrical element is then cooled down further and cut transversally at many equidistant points so as to form a plurality of elongated elements, also known as “core rods”, typically having a length greater than 1 m and a diameter of between 10 and 20 mm.
In a fifth step, each core rod is used as a substrate for a further chemical deposition process (known as “overcladding”) similar to that of the first step discussed earlier. In particular, on each core rod and through at least one burner, a plurality of chemical substances are deposited (amongst which, typically, there is silicon oxide) which will then constitute the outer portion of the optical fibre's cladding. At the end of the process a low-density final preform is obtained, from which the optical fibre will then be drawn. Before the drawing, the low-density final preform is desiccated and consolidated with the same procedures seen in the third step. In this way a vitrified final preform which is ready for the drawing process is obtained.
Various devices for manufacturing a glass (core or final) preform for optical fibres through processes of the OVD type are known. Such devices typically comprise a chemical deposition chamber inside which are housed the gripping members of the cylindrical support constituting the chemical deposition substrate for the formation of the preform, a burner which is mobile parallel to the longitudinal axis of the cylindrical support, and a suction hood positioned on the opposite side to the burner with respect to the cylindrical support and adapted to collect and remove the particulate and the exhaust chemical substances produced inside the chamber during the chemical deposition.
JP 11-1338 discloses a device for manufacturing a preform for optical fibres through an OVD process comprising a pair of burners which are mobile parallel to the longitudinal axis of a support for preform formation which rotates upon itself and a suction hood provided on the opposite side to the burners with respect to the cylindrical support and also mobile parallel to the longitudinal axis of the cylindrical support.
JP 2000-313625 discloses a device for manufacturing a preform for optical fibres, comprising a plurality of burners which are adjacent the one to the other and mobile parallel to the longitudinal axis of a support for preform formation, which support rotates upon itself. A suction hood is provided on the opposite side to said plurality of burners with respect to the cylindrical support; said hood also moves parallel to the longitudinal axis of the cylindrical support and in synchrony with said plurality of burners. In particular the hood and the plurality of burners are controlled by respective motors connected to a single motion control circuit.
U.S. Pat. No. 5,211,732 discloses a device for manufacturing a preform for optical fibres, wherein the gripping members hold the cylindrical support for preform formation in a vertical position and the chemical deposition takes place through a series of burners which substantially extends along the whole length of the cylindrical support and which is made to oscillate parallel to the longitudinal axis of the support in such a way that each burner only acts on a predetermined portion of support. The device, moreover, comprises an air circulation system comprising a honeycomb structure arranged upstream of the cylindrical supports and adapted to uniformly distribute, in the deposition area, the air which enters the chamber through a plurality of air suction members formed on the rear wall with respect to the burners, and a diffusor arranged downstream of the cylindrical supports and adapted to suck the air flows from inside the chamber. In particular, the honeycomb structure generates a plurality of air flows which are controlled so as to have substantially laminar flows distributed uniformly along the whole length of the cylindrical support and substantially perpendicular to the longitudinal axis of the support itself.
JP 2001-019463 relates to a technique for producing a porous preform for optical fibers, wherein glass particulates are blown from an oxygen-hydrogen flame burner to an horizontal axially rotating rod and are deposited thereon in a reaction vessel, and a moving stage mounted with the burner is moved back and forth between two turning points. An air exit having an exhaust hood is arranged on the side opposite to the oxygen-hydrogen burner with respect to the rod to discharge exhaust gases containing unreacted components, and the like, to the outside of the vessel. The air exit is moved in parallel to the burner via a moving stage, a guide, a motor, or the like, with a delay therefrom to efficiently introduce the exhaust gases from the burner into the exhaust hood.